The present invention relates in general to semiconductor devices and, more particularly, to adhesion to a polyimide surface by formation of covalent bonds.
Semiconductor devices are widely used in many products. An important feature of semiconductor devices is the small size of the semiconductor device and any associated module housing. As the products using semiconductor devices grow smaller, it is important to keep the dimensions of the semiconductor devices and module housing as small as possible. However, competing with this need for smaller dimensions is the increasing need for functionality that the consumers of semiconductor devices demand. The increase in functionality tends to increase the size and complexity of the semiconductor devices and the number of semiconductor devices per module.
Another significant factor is maintaining a low cost of manufacturing despite the increase in semiconductor device complexity and density. One significant cost in manufacturing a semiconductor device is the lead frame. Often, customized lead frames must be manufactured for each type of semiconductor device which is costly and time consuming. As a result, direct chip attach (DCA) type assemblies are gaining in popularity. In DCA, the semiconductor device is directly attached to a substrate, for example, a printed circuit board (PCB). DCA involves directly coupling the pads of a semiconductor device, without a lead frame, to matching contacts on the PCB. Thus the cost and size of an individual package for the semiconductor device is eliminated.
DCA is facilitated by fabricating raised metal bumps on semiconductor device pads, and mounting the device, bumps down, onto the PCB or other type of substrate. In order to protect the interconnections between the substrate and the semiconductor device from fatiguing during thermal cycling, and to environmentally protect the semiconductor device itself, a dielectric material is often applied between the semiconductor device and substrate. The dielectric material is usually a polymer, typically an epoxy resin, that forms a bond to the semiconductor device, the interconnections and the substrate.
In the prior art, when the surface passivation of the semiconductor device and/or the substrate is polyimide, adhesion of any polymer encapsulant to the polyimide surface has shown a tendency to separate during thermal cycling. Although chemical coupling agents, mechanical roughing and plasma etching have been attempted to improve the bonding, the adhesion problem continues to lead to premature separation and failure of the semiconductor device or interconnection fatigue failure.
Hence, a need exists for improved adhesion to polyimide surfaces.